Magnetoresistive sensors are typically small and can generally measure magnetic fields on the order of 0.001 gauss to 100 gauss. Also, magnetoresistive sensors are able to measure D.C. fields as well as fields having frequencies up to and exceeding a megahertz. Accordingly, magnetoresistive sensors are used in a wide variety of applications such as current sensing, proximity sensing, etc.
The magnetoresistive material used in making magnetoresistive sensors is a material whose resistance changes in the presence of a magnetic field. Permalloy, which is a nickle/iron alloy, is such a material and is often provided as a film for use in magnetoresistive sensors. The resistance of the film varies according to the square of the cosine of the angle between the magnetization direction and the current direction.
The response of a magnetoresistive material is measured as ΔR/RN, where ΔR is the change in resistance of the magnetoresistive material and RN is the nominal resistance of the magnetoresistive material. The change in the resistance ΔR of Permalloy between the point where the magnetization direction is parallel to the current direction and the point where the magnetization direction is perpendicular to the current direction is on the order of 2% of the nominal resistance of the material.
Moreover, the plot of ΔR/RN versus the angle between the magnetization direction and the current direction is bell shaped. In order to operate the magnetoresistive material on the linear part of this curve, a bias field is frequently applied to the magnetoresistive sensor. For example, either a solenoid wrapped around the magnetoresistive sensor package or a plurality of thin-film permanent magnets at the end of the magnetoresistive sensor are usually used to bias the magnetoresistive material at this linear portion.
Alternatively, instead of applying a biasing field to the magnetoresistive sensor, it is known to provide the magnetoresistive sensor with conductive barber poles. Unlike the bias field which rotates the magnetization direction with respect to the current direction, barber poles instead rotate the current direction with respect to the magnetization direction.
Magnetoresistive sensors are frequently used in Wheatstone bridges. Thus, each of the four legs of a Wheatstone bridge contains a magnetoresistive sensor. A top view of an exemplary known Wheatstone bridge 10 is shown in FIG. 1. The Wheatstone bridge 10 includes four magnetoresistive sensors 12, 14, 16, and 18. The magnetoresistive sensor 12 is formed from a Permalloy film 20 and has a set of barber poles 22 for biasing. The magnetoresistive sensor 14 is formed from a Permalloy film 24 and has a set of barber poles 26 for biasing. The magnetoresistive sensor 16 is formed from a Permalloy film 28 and has a set of barber poles 30 for biasing. Finally, the magnetoresistive sensor 18 is formed from a Permalloy film 32 and has a set of barber poles 34 for biasing.
The nominal resistances of the four legs are ideally identical so that the Wheatstone bridge 10 is balanced and has no output in the absence of a magnetic field. The use of a bias field to operate the magnetoresistive material forming the magnetoresistive sensors 12, 14, 16, and 18 in the Wheatstone bridge 10 at the linear portion of the magnetization/current curve does not upset this balance because the resistances of the four sensors change by the same amount in response to the bias field.
The use of barber poles, however, can upset this balance, such as where the barber poles are not uniformly formed over the magnetoresistive material of the four sensors. As described above, an imbalance causes the bridge to have an output even when no magnetic field is present. This output is usually referred to as offset. Accordingly, it is known to reduce this offset through the use of laser trimming. However, laser trimming adds cost to devices such as Wheatstone bridges which use magnetoresistive sensors.
The present invention is directed, at least in one embodiment, to an arrangement of barber poles which allows tighter control of offset and, therefore, reduces the need for laser trimming.